CONTRIBUTION OF SULPHUR (DIMETHYL SULPHIDE)FROM THE FRESHWATER LAKES OF THE CANADIAN BOREAL REGION.

The biogenic source strength of sulphur (Dimethyl Sulphide) from the freshwater lakes is required to assess its relative importance on a regional scale.
A two-year field study was undertaken during the ice-free seasons of 1995 and 1996. Surface lake water DMS concentration (DMS{sub(w)}) was measured in ten lakes at the Experimental Lakes Area, located in the northwestern Ontario. DMS{sub(w)} ranged from 0.1 nmol/L to 100 nmol/L during the ice-free season. Depth profiles were also taken in all of these lakes. Some vertical profiles in the water column of medium depth lakes (10-12 m in depth) showed higher DMS{sub(w)} in the hypolimnion (up to 150 nmol/L) during late summer. Lake turn over, during the autumn, resulted in consistently higher DMS{sub(w)} in these lakes. Simultaneous measurements of other chemical species such as sulphate, chla, nitrate will also be shown.
Estimated fluxes of DMS (using a Wanninkhof's model for gas exchange) varied from 0.058 <micro>mol S m{super(-2)} d{super(-1)}and 15 <micro>mol S m{super(-2)}d{super(-1)} respectively. Extrapolation of the flux estimates indicate that the lakes of the Canadian boreal region contributed biogenic sulphur of about 1.5 Gg/a. These contributions are about 83% of the annual biogenic sulphur (DMS) emissions from the terrestrial region of the Canadian Boreal Shield.